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Computational Fluid Dynamics of the Boundary Layer Characteristics of a Pacific Bluefin Tuna

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Technical Report

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Naval Undersea Warfare Center Division Newport United States

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The mechanism by which tuna achieve very fast swimming speeds is not presently understood, and may involve delay of transition or an advanced propulsion mechanism or both. The issue of whether the boundary layer on a tuna swimming at typical speeds 1 to 2 bodylengthssec is laminar, turbulent, or transitional is an open question. Using an arc-length Reynolds number ReL to estimate the nature of the boundary layer and predict when transition occurs only serves as a rough approximation. Uncertainties include the surface roughness of the skin, local favorable and adverse pressure gradients, and discontinuities such as the open mouth or juncture at the fins. The primary objectives of this project are to compute the approximate lateral location at which transition to turbulence occurs on the tuna for various swimming speeds, and to determinethe maximum speed at which laminar flow is retained on the tunas body. Two-dimensional 2D and three-dimensional 3D computer models are used to compute the boundary layer characteristics and predict the lateral location of turbulence onset. The computations cover speeds ranging from 2 to 22 ms.

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